Reduce button presses to get a specific presetted trace

Users of tracing many times uses predefined preset modes e.g.
Web Developer, Frame Viewer, Input Latency etc.
This involves pressing 'Record', then choosing the preset option
from dropdown and then pressing Record button.

Instead we can list presets as options, where clicking on the preset
starts recording with the pre-defined trace categories. Only if we
choose "manual settings" mode, we need to press Record button.

The UX changes of this CL are as per the wireframes suggested by pdr
in the document:
https://docs.google.com/a/chromium.org/drawings/d/1fTlQyI2vdJoVzMqmdX9qRQsZIdJ1zzz_FCCEKfJ5S8M/edit

BUG=603

R=nduca,dsinclair,pdr
nit: else on the same line as the }